1. Field of the Invention
The invention relates to a method for forming an air gap structure on a substrate and, more particularly, to a method for integrating an air gap structure with a substrate.
2. Description of Related Art
As is known to those in the semiconductor art, interconnect delay is a major limiting factor in the drive to improve the speed and performance of integrated circuits (IC). One way to minimize interconnect delay is to reduce inter-connect capacitance by using low dielectric constant (low-k) materials as the insulating dielectric for metal wires in the IC devices. Thus, in recent years, low-k materials have been developed to replace relatively high dielectric constant insulating materials, such as silicon dioxide. In particular, low-k films are being utilized for inter-level and intra-level dielectric layers between metal wires in semiconductor devices.
Additionally, in order to further reduce the dielectric constant of insulating materials, material films are formed with pores, i.e., porous low-k dielectric films. Such low-k films can be deposited by a spin-on dielectric (SOD) method similar to the application of photo-resist, or by chemical vapor deposition (CVD). Thus, the use of low-k materials is readily adaptable to existing semiconductor manufacturing processes. However, low-k films and, more specifically, porous low-k films have suffered integration problems including, but not limited to, poor thermal and mechanical performance, copper migration, damage during pattern etching, etc. As a result, for example, the integration of low-k films and porous low-k films has required the use of capping layers having a higher dielectric constant, as well as the development of techniques to restore the dielectric constant of these films resulting from carbon depletion at exposed surfaces, and other techniques to seal exposed pores in the surface of porous low-k films.
Furthermore, in yet another attempt to reduce the dielectric constant of insulating materials, air-gap structures are contemplated. According to one approach, air-gap structures are formed by depositing a sacrificial material on a substrate and then depositing a bridging material over the sacrificial material. Thereafter, at a later point in the device manufacturing process following metallization and planarization, the sacrificial material is decomposed and removed in order to leave a gap or void in its absence. Conventionally, the sacrificial material is removed using a chemical or thermal process. Thus, the sacrificial material plays the role of template or “void precursor”, wherein the void is formed upon decomposition of the sacrificial material by thermal treatment and diffusion of the decomposition products out of the multilayer assembly. Thermally degradable polymers have been a preferred choice for use as a sacrificial material.
However, despite the promise of superior electrical performance by this approach, thermally degradable materials still face formidable challenges including, but not limited to, solvent resistance, thermal stability, and mechanical strength. For example, numerous acids, bases and organic solvents are utilized in IC manufacturing, and the sacrificial materials must retain their original dimensions regardless of the presence of these chemicals. Dissolution of sacrificial materials or swelling should be rigorously controlled or excluded. Additionally, for example, chemical vapor deposition of barrier layers on sacrificial materials is anticipated to take place at a substrate temperature between about 250 degrees C. and about 320 degrees C. Hence, the sacrificial material must be thermally stable in this temperature range.
Additionally yet, for example, the mechanical properties of the sacrificial material, such as hardness and Young's modulus, should be sufficiently high to withstand chemical mechanical planarization (CMP) and flip chip bonding. Furthermore, the adhesion of the sacrificial layer to the underlying materials on the substrate should be sufficient to withstand subsequent forces acting thereon. Further yet, for example, the fraction of residue remaining on the substrate following decomposition of the sacrificial material should be minimized to guarantee proper electrical performance. Even further yet, for example, the sacrificial material should be decomposed under conditions not suitable for curing the bridging material.